Spatial light modulators (SLMs) are in wide use in display systems in part due to having the benefit of high resolution while consuming lower power and having less bulk than conventional Cathode Ray Tube (CRT) technology. One type of SLM display is the digital micromirror device(DMD). Some projection display systems use microdisplays, such as DMDs or deformable micromirrors, to generate an image on a display plane. In general, a microdisplay in a projection display system is used to create a miniature version of the image to be displayed, and optical lenses and elements are used to project an enlarged version of the image on the display plane. DMDs are used in Texas Instrument DLP® technology as optical switches or transmitters for television (TV) and projection systems. DMDs are optical semiconductor devices having an array of thousands or up to millions of micromirrors that are switched on or off at varying frequencies, forming a digital image. Some systems may have a single DMD, whereas other systems may include three DMDs, as examples. Systems that utilize DMDs have a high fidelity and improved picture quality.
Each micromechanical display element, in a DMD array, has a mirror that is individually addressable by an electronic signal. Depending on the state of its addressing signal, each mirror tilts so that it either does or does not reflect light to the image plane. The mirrors may be generally referred to as display elements, which correspond to the pixels of the image that they generate, in this known structure. Generally, displaying pixel data is accomplished by loading memory cells connected to the display elements. The display elements maintain their ON or OFF state for controlled display times.
Intermediate levels of illumination, that is levels between white (ON) and black (OFF), may be achieved, by pulse-width modulation (PWM) techniques, in one known method. The basic PWM scheme involves first determining the rate at which images are to be presented to the viewer. This establishes a frame rate and a corresponding frame period. For example, in a standard television system, images are transmitted at 30 frames per second, and each frame lasts for approximately 33.3 milliseconds.
In a simple example, assuming n bits of resolution, the frame time is divided into 2n−1 equal time slices. Upon establishing these times for each pixel of each frame, pixel intensities may be quantized, such that black is zero time slices, the intensity level represented by the least significant bit (LSB) is one time slice, and maximum brightness is 2n−1 time slices, where n is the bit resolution. Each pixel's quantized intensity determines its ON-time during a frame period. Thus, during a frame period, each pixel, with a quantized value of more than zero, is ON for the number of time slices that corresponds to the pixel's intensity. The viewer's eye integrates the pixel brightness so that the image appears the same as if it were generated with analog levels of light.
To address DMDs, PWM calls for the data to be formatted into bit-planes, each bit-plane corresponding to a bit weight of the intensity value. A bit plane of a digital discrete signal (e.g., of an image) is a set of bits having the same position in the respective binary numbers. For example, for 8-bit data representation, there are 8 bit-planes, wherein the first bit-plane contains the set of the MSBs and the 8th bit-plane contains the set of the LSBs. The MSB bit-plane generally gives the roughest but most critical approximation of pixel values of an image, and the less significant bit plane, the less is its contribution to the image value approximation. Thus, adding a bit-plane generally gives a better approximation of the image. Each bit-plane from the LSB to the MSB has twice the bit weight of the previous bit-plane.
Thus, if each pixel's intensity is represented by an n bit value, each frame of data has n bit-planes. Each bit-plane has a 0 or 1 value for each display element. Each bit-plane may be separately loaded and the display elements may be activated according to their associated bit-plane values. For example, the bit-plane representing the LSBs of each pixel is displayed for one time slice, whereas the bit-plane representing the most significant bit (MSB) is displayed for 2n/2 time slices.
Turning to FIG. 1A for a simplified version of this concept, a prior art DMD mirror 102 is shown with a corresponding graph of light intensity versus time in FIG. 1B. For this illustration, the intensity resolution n=2 is used, therefore an integrated time period tp, 2n−1 is equal to three time slices. Therefore each of the frames represented by time period, t1, t2, and t3, is subdivided into three time slices a, b, and c. The MSB occupies time slices a and b, while the LSB occupies time slice c. In time period t1, the bit value is 00, therefore zero time slices are illuminated for time period t1, and to the eye, time period t1 is dark. In time period t2, the bit value is 11, so the mirror is illuminated for three out of three time slices, and to the eye, time period t2 is at a maximum or 100% bright. In time period t3, the bit value is 01, so the mirror is dark for two time slices and then illuminated for one time slice, for time period t3, therefore, the eye perceives time period t3 as 33% as bright as time period t2.
In this simplified prior art example there are no illuminated integrated times that are less than 33% bright. In application, however, DMD devices typically control brightness with more than two bits resolution, thus there are many more time slices per frame. However, even in known more complex DMD devices the lowest intensity displayable is governed by the duration of a time slice. In other words, depth of intensity is controlled by timing.
Although the DMD display bit depth is adequate for some applications, an enhanced display bit depth is desired. What is needed then is a new structure and method of providing a lower illumination display bit depth.